This invention relates to piezo-electric material, more particularly to piezo-electric material with a high piezo-electric modulus, and having excellent flexibility and moldability.
Heretofore, it is well known that crystals which do not have a center of symmetry show piezo-electricity and inorganic materials such as rock crystal, Rochell salt, and lead zirconate-lead titanate ceramics are usually utilized as piezo-electric material. These materials have high piezo-electric modulus, but they are difficult to be formed. Therefore it is difficult to obtain thin piezo-electric material with pliability. On the other hand, it is recognized that some high polymer materials such as natural high polymer as cellulose and protein, and stretched film of poly-.gamma.-methyl-L-glutamate synthetic high polymer, and the like, show piezo-electricity. Besides it is known that some electrets of synthetic high polymers, for example, electrets obtained by a manufacturing method which comprises stretching films such as polyvinyl fluoride, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile polycarbonate, and the like by several times at a temperature approaching the softening temperature, heating from a room temperature under a condition of applied electric field, and cooling, show piezo-electricity. Organic piezo-electric materials obtained in the way described above have good flexibility and pliability, but they have anisotropy of piezo-electric moduli because stretching treatment is done, and there are many electrets whose piezo-electric moduli are not very high and variable according to temperature or humidity. For example, even a piezo-electric modulus in stretching direction (d.sub.31) of electretized polyvinylidene fluoride which has the highest piezo-electric modulus is only 4.0 .times. 10.sup.-7 (CGS esu). Further, as they are stretched, it is very difficult to make molded goods with complicated form.